Fuel tanks intended for motor vehicles, especially plastic fuel tanks, have to meet specifications that specify maximum permissible amplitudes of deflection on their lower skin. The deflections stated in these specifications usually have to be met during ageing tests in which the tank contains a certain quantity of fuel for a given period of time (typically several weeks) and at temperature (usually 40° C.). The purpose of these specifications is to ensure that vehicles maintain their road clearance and to prevent the skin of the tank from coming into contact with hotspots of the vehicle.
At the present time, hybrid vehicles and particularly cars operating in electric mode only, are characterized by a significant reduction of the volumes of air for purging the canister. In the context of the development of fuel systems for applications of this type, a pressurization of the tank is envisaged, since the generation of petrol vapors decreases as a function of the pressure. At pressures of 350 to 450 mbar, vapor generation is almost eliminated. Thus the canister is no longer affected by changes in the surrounding temperature. On the other hand, the mechanical strength/creep resistance of the tank must be increased since the deformations resulting from the pressurization are added to the deformations induced by the weight of the fuel.
Solutions have been proposed in the prior art with a view to enhancing the mechanical strength (including the creep resistance) of fuel tanks.
For instance, patent application WO 2010/122065 in the name of the Applicant describes a fuel tank having a lower wall, an upper wall and at least one reinforcing element connecting these two walls, said reinforcing element comprising a hollow plastic pillar, at least one part of which being a constitutive element of an accessory that has an active role in the tank.
Although the solution set forth above can work in some circumstances, there are nevertheless specific cases in which it can lead to problems owed to the fact that the top and bottom of the tank are not parallel due to:                blow molding tolerances;        deformations during part cooling;        deformations during thermal expansion;        deformations of tank shells not identical in top and bottom tank surface;        a specific non-parallel design of the top surface and the bottom surface of the tank.        
In these cases, a traditional reinforcing element (either in the shape of a pillar or not) tends to concentrate stresses into localized regions of the reinforcement and/or the interface. More specifically, since no movement between the two opposite tank surfaces is allowed (this design not allowing any misalignment of the linked surfaces), all stresses are taken by the reinforcing element which in certain cases could create too high stress concentrations leading to break of the internal reinforcement after a long pressurized situation.